By now you’ve heard about the perks that come with working in Silicon Valley. Free lunch, 20 percent time -- that’s the work time you can use to pursue independent projects.

Well, another perk? A private bus that picks you up in your neighborhood in San Francisco and shuttles you down to your corporate campus about an hour south in the suburbs of Silicon Valley.

During rush hour in San Francisco, you see them everywhere, said Eric Rodenbeck, the creative director of Stamen Design in the Mission District of San Francisco.

“They’re just so big," Rodenbeck says. "These buses are two stories high and they’re barrelling down residential streets, and no one knows where they’re going except the people who are on them.”

Rodenbeck is talking about the private shuttle buses that run up and down the Peninsula. They look like fancy tour buses. Google’s buses are white. Facebook’s are a sleek blue. But beyond that, they’re sort of a mystery to most San Franciscans.

“You know it’s almost like this masonic ritual,” Rodenbeck says. "If you've got the key, this whole other city layer unlocks itself to you. And that’s the kind of urban puzzle we like to solve."

But getting the data wasn’t easy. The tech companies don’t comment on the buses. They don’t tell you where they stop or how many people ride on them. But in the era of big data, the information was easy enough to find.

“Even though the companies might not have wanted their locations public, we started looking around and we realized on Foursquare -- if you typed in “shuttle” and “google” or “shuttle” and “apple” all these locations came up because their employees were checking in at those bus stops,” Rodenbeck says.

Stamen also hired bike messengers to follow the buses. And then they had people just sit at a cafe on the corner of 18th and Dolores and count the people getting on and off the buses.

I checked out the Google bus stop a little after 7 a.m. one rainy morning and the “G-bus,” as the display on its windshield reads, was already picking of Googlers. For the next few hours, the buses would arrive in 15-20 minute intervals and a steady stream of 20-30 somethings, holding coffee cups and wearing sneakers and backpacks, would get on board.

It might have been the early morning hour or the rain but few people were willing to talk. When I approached a group of 20-somethings and asked them about the bus, they said they couldn’t talk because Google was in "a quiet period." A quiet period is when a company can’t say anything that might affect its stock price, and that was the nicest response I got until I met 35-year-old Tanya Birch, who works on the Google Earth outreach team. I asked her what it’s like on the bus.

There’s also free Wi-Fi on the shuttles, and Birch said it's basically another hour of work.

The tech world is driven by young, educated largely urban workers. But companies like Facebook, Google and Apple are located in the suburbs of Silicon Valley, which is about an hour south of the San Francisco.

“I think a lot of young people who work at the tech companies they want the city life they want something that’s fun and entertaining, and you don’t get that in the suburbs,” Birch said.

So, to compete for that talent pool, big tech companies have to provide transportation. Rodenbeck says he expected to find the shuttles in the city’s hip, young neighborhoods.

“What we were surprised to learn is that the network is much more extensive than that,” says Rodenbeck.

When the map was finished, Stamen counted busses from Apple, eBay, Electronic Arts, Facebook, Google and Yahoo, and they found the buses ran through almost every neighborhood in San Francisco. Stamen estimates that about 14,000 people ride the private shuttle buses every day.

Rodenbeck says he thinks the locations are secret because the companies are “sensitive to this idea that they are funding a change in the infrastructure in San Francisco without it being regulated.”

“The increase in employer buses has sparked some reaction from residents,” Robbins says.

He says that since tech companies contract out the work to private bus companies, which are regulated by the state, the city has little say in what they do.

But Robbins says the agency has fielded complaints that the the private shuttle buses, which often stop at public bus stops, are causing delays and traffic.

Another impact is rising real estate prices, says Amanda Jones, a realtor in San Francisco for nearly a decade. Today, about half her clients work in the tech industry.

“Unquestionably the shuttle stops are transforming real estate values,” Jones says. “When I interview new clients, we get out the real estate map and they want to show me where their corporate shuttles are. I recently sold a house. He does trading for Google and gets in early in the morning. Literally, if it wasn’t five blocks from a shuttle stop, we didn’t look at it.”

Jones says even fixers-uppers and homes with shaky foundations are selling for a premium if they’re located near a private shuttle bus stop.

“They have so little time to have with family and their friends they want to go home and be able to walk to the restaurant and not be stuck in their car for two hours,” says Jones.

Jones says she gets it because until someone comes up with an app that can beam you to work, the private shuttle bus is as close as you get.

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